Single crystal silicon (Si) is conventionally used as a material for a power semiconductor device controlling high current and high voltage (a power device). Various types of power semiconductor devices exist and each is used for a purpose suitable therefor. For example, a bipolar transistor and an insulated gate bipolar transistor (IGBT) can respectively handle a high current density but these transistors are incapable of high speed switching. Frequency limits for the bipolar transistor and the IGBT are on the order of several kHz and about 20 kHz, respectively. On the other hand, although a power MOSFET (Metal Oxide Semiconductor field effect transistor) cannot handle high current, this transistor can switch at high speeds up to several MHz.
In the market, demand is strong for a power semiconductor device that can cope with high current and high speed, and efforts have been made to improve the IGBT, the power MOSFET, etc. Therefore, at present, the performance of power devices substantially reaches the theoretical limit decided by materials.
FIG. 1 is an explanatory diagram of a cross-sectional view of a common MOSFET. FIG. 1 depicts a cross-sectional view of a typical (common) MOSFET as a power device capable of coping with a high current and a high speed. In FIG. 1, an epitaxially-formed, low concentration N−-drift layer b is disposed on a front face side of a substrate a. A P-base layer c is further formed in a surface layer on the front face side of the low concentration N−-drift layer b. A high concentration N+-source layer d is selectively formed in the surface layer on the front face side of the P-base layer c. A gate electrode f is formed through a gate insulating film e on the N−-drift layer b, the P-base layer c, and the high concentration N+-source layer d. A drain electrode g is formed on the side of a back face of the substrate a.
A MOSFET has recently attracted attention. FIGS. 2, 3, and 4 are explanatory diagrams depicting a cross-sectional view of the structure of a conventional silicon superjunction MOSFET. FIGS. 2 to 4 depict, as a superjunction MOSFET, a cross-sectional view of the structure of a typical device. The theory of the superjunction MOSFET was reported by Fujihira, in 1997 (see Non-Patent Literature 1 below) and this MOSFET was established as a product called “CoolMOS” by Deboy, et al, in 1998 (see Non-Patent Literature 2 below). The ON-resistance of the superjunction MOSFET can be improved significantly without degrading the breakdown voltage between the source and the drain by forming a P-layer in a columnar structure in a vertical direction in the N−-drift layer.
Investigations concerning the material from the viewpoint of the power semiconductor device has been conducted. For example, as Shebnai, et al, reported that SiC is excellent with regard to low ON-voltage, high speed, and high temperature properties and therefore, SiC has recently attracted attention especially for next generation power semiconductor devices (see Non-Patent Literature 3 below).
In view of SiC attracting attention for next generation power semiconductor devices, it can be said that SiC is a highly stable material chemically, has a wide band gap of three eV, and can be used very stably for a semiconductor even at high temperatures. Further, since SiC has attracted attention for next generation power semiconductor devices, it can be further said that the critical electric field strength thereof is higher than that of Si by one or more digits.
The material performance of SiC exceeds the material performance limits of silicon and therefore, the use of SiC for power semiconductors is expected, especially for MOSFETs. In particular, there are high expectations related to the low ON-resistance of SiC and for a vertical SiC-MOSFET that has even lower ON-resistance, while maintaining the high breakdown voltage.    Patent Document 1: U.S. Pat. No. 7,923,320    Non-Patent Literature 1: Fujihara, JJAP, vol. 36, Part 1, No. 10, p. 6254, 1997    Non-Patent Literature 2: Deboy, et al, IEEE IEDM 1998, p. 683    Non-Patent Literature 3: Shenai, IEEE Transcation on Electron Devices (Vol. 36, p. 1811), 1989